1. Field of the Invention
The present invention relates to a system and method for controlling automatically implemented range shifting in a compound transmission having a main section shifted by a manually operated shift lever. In particular, the present invention relates to a transmission control system and method for automatic range shifting that selects and initiates a range shift in response to a selected target gear ratio.
2. Background Art
Controller-assisted, compound, manually-shifted, mechanical transmissions are widely known in the field of heavy-duty vehicles, such as transport trucks. Typically, these transmissions include a main section shifted directly or remotely by a manual shift lever and one or more serially connected auxiliary sections. The auxiliary sections are usually shifted by a slave actuator and are either pneumatically, hydraulically, mechanically and/or electrically operated in response to manual operation of one or more switches.
In addition to the operation of various switches, drivers of heavy-duty vehicles must be attentive to various vehicle and driving conditions to properly engage transmission shifts without causing damage to the vehicle. For example, while driving a heavy-duty vehicle, the driver must watch the vehicle speed, the road conditions, operate the shift lever, control a splitter select switch to engage a particular gear ratio, and control a range select switch to toggle between at least a high range and a low range. As such, ensuring proper transmission shifts is a complicated task, especially for the inexperienced driver.
Transmissions having sensors, actuators, and a microprocessor-based controller to automatically assist manual shifting are well known in the art. These transmissions are capable of detecting an intent to shift signal and can reduce driveline torque in response to an intent to shift signal. Some prior art transmissions can sense an intended target gear ratio and consequently synchronize transmission components to engage the target ratio.
Prior art devices with lever or switch position initiated automatic range shifting, while adding conveniences, are not totally satisfactory. Automatic range shifting is generally initiated as the shift lever actuates a fixed position sensor or switch. The lever position at which automatic range shifting is initiated is fixed. For example, as the shift lever is moved into a transition area or band from one leg of an H-shift pattern to a higher leg, it crosses a point and actuates a fixed sensor to cause automatic implementation of a range shift. One example of this range shifting method may be seen by referring to U.S. Pat. No. 5,911,787, which is assigned to applicant's assignee. However, this method for automatic range shifting cannot be used with a repeated H-shift pattern. In addition, prior art devices do not prevent or inhibit non-synchronous shifting with a high degree of reliability. When the vehicle is operating in high range and an operator decides to shift the vehicle into low range at the wrong time, an engine overspeed condition may occur if the operator is not careful.